This invention relates to optically addressed light valves and more specifically to real-time reflective light valves without optical blocking layers.
With the rapid advance in imaging process techniques, several types of optically addressed light valves are now practical. Desirably such light valves should be simple, i.e., have the least possible number of layers, and permit readout during the write-in cycle. The intensity of the readout light should exceed the write-in intensity, which preferably is small, by many orders of magnitude.
Several types of light valves are described in the patent and scientific literature. In general, such light valves consist of a photoconductor in series with an electro-optic medium such as a liquid crystal, ferroelectric or the like. The photoconductor is under electrical bias and light imagewise incident on it effectively causes its resistance to decrease in the exposed area. This results in imagewise changes in the field across the electro-optic medium which in turn can be read out or projected.
It has heretofore been accepted that prevention of image washout by the readout light would require either readout with light of wavelengths to which the photoconductor is insensitive or, alternately, insertion of an electrically conductive optical blocking layer between the electro-optic medium and photoconductor. Both cases are discussed in the literature. For the type of light valve in which the readout light is of a wavelength which does not affect the photoconductor see for example an article in Applied Physics Letters for July 15, 1970 by J. D. Margerum, J. Nimoy and S. Y. Wong, entitled "Reversible Ultraviolet Imaging with Liquid Crystals." The photoconductor is zinc sulfide, the electro-optic medium a liquid crystal and the electro-optic effect dynamic scattering. The light valve is addressed with ultraviolet light to which the photoconductor is sensitive and read out with visible light to which it is insensitive.
Another example can be found in U.S. Pat. No. 3,592,527 by Gary H. Conners and Paul B. Mauer entitled "Image Display Device," who use poly-n-vinylcarbazole or triphenylamine as the photoconductor, and liquid crystals as the electro-optic media. The light valve is addressed with ultraviolet light and read out with light not containing ultraviolet. Examples of the use of optical blocking layers to prevent readout light from reaching the photoconductor can also be found in the literature. For example, an article by J. R. Maldonado and L. K. Anderson in IEEE Transactions on Electron Devices, Vol. ED-18, No. 9, 1971 entitled "Strain-Biased Ferroelectric-Photoconductor Image Storage and Display Devices Operated in a Reflection Mode" describes an "opaque resistive sea to provide complete light isolation between the writing and reading sides of the device." The light valve consists essentially of a ferroelectric ceramic whose optical properties are controlled by an imagewise exposed photoconductor.
More recently, in an article entitled "AC Liquid-Crystal Light Valve" by T. D. Beard, W. P. Bleha and S. Y. Wong which appeared in Applied Physics Letters of February 1, 1973, cadium sulfide is used as the photoconductor, and a liquid crystal as the electro-optic medium. To separate the write-in light from the readout light, an optical blocking layer of cadmium telluride is used in conjunction with a dielectric mirror.
In new and growing areas of technology, new methods, apparatus, compositions and articles of manufacture are often discovered for the application of the new technology in the new mode.